Description: (Adapted from applicant's description) This project proposes to study the development of an understudied, yet central, aspect of human cognitive activities: mathematics. In adults, two neural circuits involving mathematics have been described: the intraparietal cortex is particularly called for during approximation tasks; when exact calculation is required adults rely on a linguistically coded memory of number facts involving a circuit of language-based areas including the inferior aspect of the left frontal lobe and the angular gyrus. The present project aims to study how these two circuits develop or fail to develop in childhood. Various types of acalculia have been described in adults, but a corresponding typology and its underlying neural bases remain to be firmly established in children. Based on the previous work of these investigators, they expect that there should be at least two large categories of dyscalculias, those that are derivative of an associated language deficit, and those that are solely due to a central deficit of the non-verbal quantity representation and ensuing loss of number sense. They plan to test this hypothesis by imaging the activation of number processing circuits in a large group of children with normal or impaired numerical development, and with or without associated language deficits. This project is designed to characterize the circuits for exact and approximate calculation in 10-12 year-old children; to establish the patterns of brain activation associated with good and poor arithmetic performance; to test the hypothesis that there are at least two subtypes of developmental dyscalculia, one associated with a language deficit and left-hemispheric dysfunction, and the other associated with normal language and with uni- or bilateral parietal dysfunction; and to test the hypothesis that the role of the parietal lobe in quantity representation is partially under genetic control by testing children with Turner's syndrome; and to measure the onset of automaticity of access to numerical quantity by imaging the evolution of a number-size interference test in a longitudinal group of children between first and third grade.